When producing firearms ammunition, care must be taken that a maximum chamber pressure of the corresponding firearm is not exceeded when a cartridge is fired therefrom. In the U.S., ANSI/SAAMI standard Z299.2:1992 sets forth the maximum average chamber pressure for various firearms, including shot guns. SAAMI refers to the Small Arms and Ammunition Manufacturers' Institute, which is an organization that develops firearms and ammunition standards, coordinates technical data, and promotes firearms safety. Conventionally, shot shells are designed so that when the shot shell is fired, such as by a shot gun, the pressure generated within the shot gun's chamber does not exceed the maximum chamber pressure.
As illustrative, non-exclusive examples, Table 1 presents SAAMI pressure specifications for a variety of gauges of shot shells.
TABLE 1SAAMI Shot Gun Pressure Specifications(piezoelectric measurements)Maximum Average Maximum AverageCartridgePressure (psi)Pressure (bar)10 gauge11,000 (all)758.4 (all)12 gauge11,500 (all but 3½ 792.9 (all but 8.9 inch mag)cm mag)12 gauge 3½″ mag14,000965.316 gauge11,500 (all)792.9 (all)20 gauge12,000 (all)827.4 (all)28 gauge12,500 (all)861.8 (all) .410 Bore 2½″12,500861.8 .410 Bore 3″13,500930.8
However, there is a competing demand for shot shells that, when fired, generate higher pellet velocities. One way to increase the velocity of the pellets fired from a shot shell is to increase the amount of propellant (i.e., smokeless powder or other highly combustible charge) in the shot shell. Another way is to select a particular type of propellant, such as an easier-to-ignite propellant and/or faster burning propellant, that generates more pressure within the shot gun's chamber when the shot shell is fired, with this increased pressure typically correlating to an increased velocity of the shot pellets.
A conventional approach to balancing velocity and pressure is to utilize slower burning propellants that will generate a lower maximum chamber pressure. However, a drawback of this approach, especially in colder climates, is that the slower burning propellants often are more difficult to ignite, and therefore may result in shot shells that do not fire reliably.